Let me know if I should use different ranges for test data/any other changes needed, happy to fix

Thanks!

I think that if one can include timedelta as well that would be great. That is, plotting the timedelta, rather than adding it (which is OK as such, but the plotting is still datetime).

(Do you agree with me that it seems like it is not properly working for dates? Not relevant for the test case writing, but these tests show that it doesn't work?)

Or, thinking twice, maybe one will have to use timedelta to get something sensible "log-like"? The "log-aspect" of the contemporary dates are probably rather small.

Timedeltas do not have first class support currently (see #8869 and linked PRs from there). They are used only when added to values.

Yes, log/semilog/etc axes with datetimes do not make much sense, as they are based on the 1970-01-01 zero mark for computing the scale, and so you are rarely working on multiple orders of magnitude where log scaling makes sense (and even when you are, the zero mark is completely arbitrary and unlikely to be related to your data).

Given that the implementation of semilog[xy] is really just some kwarg management over set_[xy]scale plus plot, the latter of which is already tested with datetimes and the former of which is tested pretty thoroughly independently, I think my vote would be to just omit the tests from the datetime file.

If we were to keep it (which, again, I'd argue we likely don't), just to prove that nothing about units prevents it from being applied, this is actually the case where using dates closer to 1970 (and spanning several decades, most likely) is likely beneficial, as it would make the scale transform visually apparent.